1. Statement of the Technical Field
The present invention relates generally to Electronic Article Surveillance (“EAS”) detection systems. More particularly, the present invention relates to implementing systems and methods for adaptively controlling alarm issuance.
2. Description of the Related Art
EAS detection systems generally comprise an interrogation antenna for transmitting an electromagnetic signal into an interrogation zone, markers which respond in some known electromagnetic manner to the interrogation signal, an antenna for detecting the response of the marker, a signal analyzer for evaluating the signals produced by the detection antenna, and an alarm which indicates the presence of a marker in the interrogation zone. The alarm can then be the basis for initiating one or more appropriate responses depending upon the nature of the facility. Typically, the interrogation zone is in the vicinity of an exit from a facility such as a retail store, and the markers can be attached to articles such as items of merchandise or inventory.
One type of EAS detection system utilizes AcoustoMagnetic (“AM”) markers. The general operation of an AM EAS detection system is described in U.S. Pat. Nos. 4,510,489 and 4,510,490, the disclosure of which is herein incorporated by reference. The detection of markers in an AM EAS detection system by pedestals placed at an exit has always been specifically focused on detecting markers only within the spacing of the pedestals. However, the interrogation field generated by the pedestals may extend beyond the intended detection zone. For example, a first pedestal will generally include a main antenna field directed toward a detection zone located between the first pedestal and a second pedestal. When an exciter signal is applied at the first pedestal it will generate an electro-magnetic field of sufficient intensity so as to excite markers within the detection zone. Similarly, the second pedestal will generally include an antenna having a main antenna field directed toward the detection zone (and toward the first pedestal). An exciter signal applied at the second pedestal will also generate an electromagnetic field with sufficient intensity so as to excite markers within the detection zone. When a marker tag is excited in the detection zone, it will generate an electromagnetic signal which can usually be detected by receiving the signal at the antennas associated with the first and second pedestal.
One limitation of EAS detection systems is the detection of tagged items in the back-field area behind the pedestal antennas. Tag detection in this area will trigger alarms that are considered false, since the customer carrying the merchandise is not exiting the store. One method used to reduce back-field is to change the antenna's transmit and receive patterns from transceivers (transmit and receive simultaneously) to transmit or receive only. This method is effective in reducing back-field alarms. However, this method reduces the systems performance in the valid detection area. Other methods which compare received amplitudes between multiple antennas have been successful in reducing back-field false alarms. But, these algorithms could be unreliable due to their dependence on noise amplitudes.